1. Field of the Invention
The invention relates in general to the transmission of parallel channels in a code division multiple access system. In particular, the invention relates to the transmission of two channels the data communications requirements of which differ from each other as regards e.g. the amount of data transmitted or data integrity.
2. Prior Art
In their operation, terminals, such as mobile phones, in cellular radio systems need to transmit both payload, or user, data and various control data which there are usually considerably less than user data or which have different quality requirements as regards the integrity of transmitted information. Control and user data are transferred in logically separate channels and it is known several methods for multiplexing those channels into a common physical radio-frequency channel. It is usual to arrange the information transmitted by a radio apparatus into frames in which the control data and user data are located in temporally separable frame components, i.e. multiplexed in the time domain. This kind of transmission method is poorly suited to so-called discontinuous transmission (DTX) if the transmission of control data has to be continuous because of the nature of said data. In discontinuous transmission, the transmission of user data is interrupted for the moments when there is no actual information to be sent (e.g., when the user of a mobile phone stops talking during a call). In code division multiple access (CDMA) systems, however, it is usually desirable to maintain the connection by always sending at least some control data; applying DTX requires pulse-type transmission within a frame.
In systems employing code division multiple access it is known to process control data and user data in two different code channels as shown in FIG. 1. At the same time, FIG. 1 also shows other known ways to combine different logic channels in one transmission. The arrangement according to FIG. 1 is known e.g. from patent document FI 97837 which has the same applicant as this patent application. Line 10 represents a transmitted bit stream which is not very error critical but in which a maximum of 10.sup.-3 bit error ratio (BER) is allowed, and line 11 represents an error critical bit stream in which the BER has to be smaller than 10.sup.-6. In order to achieve a better bit error ratio the bit stream of line 11 is Reed-Solomon coded in block 12 and interleaved in block 13. Bit streams from lines 10 and 11 are combined in block 14 and certain tail bits are added to them in block 15 whereafter the resulting combined bit stream is convolution coded in block 16. Line 18, the bit stream of which is not error correction coded nor convolution coded, is then multiplexed in block 17 onto the same code channel. To achieve the desired symbol rate, symbol repetition in block 19 and interleaving in block 20 are used if necessary. Spreading is carried out in a coding element 21 using PN1 code, whereafter the resulting symbol stream is taken to the I branch of a radio-frequency block 22 to produce a radio-frequency transmission together with the lower code channel, to be taken to an antenna 23.
Frame control header (FCH) bits carrying information on the lower code channel are taken via line 24 to a coding block 25 and therefrom via symbol repetition 26 and interleaving 27 to block 28 where reference symbols 29 needed for synchronising the receiver as well as the power control (PC) symbols 30 are added to the symbol stream. A coding element 31 performs spreading using PN2 code, which is different from the aforementioned PN1, whereafter the timing of the lower code channel with respect to the higher one is adjusted suitable by a delay element 32 before the symbol stream is taken to the Q branch of the radio-frequency block 22 to produce a radio-frequency transmission together with the higher code channel, to be taken to an antenna 23. The delay generated by the delay element 32 may also be 0, in which case quadrature phase shift keying (QPSK) modulation is used.
In a radio apparatus according to FIG. 1, it is possible to use on the lower code channel, due to a lower bit rate, a lower power level than on the higher code channel, thus saving electric power. In small-sized cellular radio system terminals, power saving in transmission is advantageous both to lengthen the discharge time of the batteries and to limit the general noise level of the system. However, the arrangement according to FIG. 1 is not optimal from the standpoint of using different power levels because of a power amplifier (not shown) in the radiofrequency block 21 and distortion occurring in it. RF amplifiers do not behave in a linear fashion when operated near the saturation region of the amplifier. Especially in the case of modulation methods with wide amplitude variation the intermodulation products generated in the amplifier should be reduced by operating the amplifier in a so-called backed-off mode, which means the amplifier input power must be decreased compared to the power that would drive the amplifier into saturation. The resulting decrease in output power is called the output back-off (OBO). The bigger the OBO, the poorer the amplifier's efficiency which is calculated as the ratio of RF power produced to DC power consumed. In the arrangement according to FIG. 1, the OBO is proportional to the power difference of the code channels so that decreasing the power level of the lower code channel with respect to the power level of the higher code channel increases the OBO.